The present invention relates to moulded plastics materials containing filler particles.
Polymeric materials may be formed in several different ways. Acrylic sheet materials are particularly useful for forming into articles such as architectural cladding, bathtubs, shower enclosures, panelling etc. Filled curable moulding compositions based on acrylic polymers which comprise a polymerisable syrup of polymer in monomer, usually containing a high proportion of filler particles are widely used for moulding into kitchen sinks etc. The cured articles have superior properties imparted by the fillers in the moulding composition and the high molecular weight and crosslinking of the polymer. Although the casting process produces a very good product it is relatively expensive because each article must be made separately on a batch basis. Thick sheets of cured acrylic composite material may also be made by a continuous casting process. An example of such a sheet is xe2x80x9cCorianxe2x80x9d(trademark) sold by DuPont. These materials are useful for making articles such as work surfaces, however they are rigid and not easily shapeable and usually must be cut and glued together to form 3-dimensional shapes. Also the continuous casting process requires very specialised processing equipment since the curing of cast syrup must be done under controlled conditions to produce optimum product quality. There is therefore a need for a polymeric material which has some of the benefits of filled acrylic composite materials but which may be shaped by thermoforming techniques. There is also a need for a method of making polymeric composite articles using cheaper processing methods.
According to the invention a process for forming an acrylic material comprises preparing a mixture comprising:
(i) 20-89 wt % of an acrylic polymer which is a copolymer containing 50-99% wt of methyl methacrylate units and 1-50 wt % of a copolymerisable alkyl acrylate;
(ii) 10-80 wt % of a mineral filler composition selected from the group comprising alumina trihydrate, magnesium hydroxide, talc and barium sulphate;
(iii) 1-40 wt % of decorative particles;
and melt-moulding the mixture at a temperature above 150xc2x0 C. to form a solid article.
According to a second aspect of the invention we provide a melt-mouldable composition comprising:
(i) 20-89 wt % of an acrylic polymer which is a copolymer containing 50-99% wt of methyl methacrylate units and 1-50 wt % of a copolymerisable alkyl acrylate or other methacrylate;
(ii) 10-80 wt % of a mineral filler composition selected from the group comprising alumina trihydrate, magnesium hydroxide, talc and barium sulphate; and
(iii) 1-40 wt % of decorative particles.
The acrylic material may be further shaped by thermoforming techniques into useful articles such as contoured worktops or vanity unit surfaces, especially when the material is in the form of a sheet. Also multi-layer structures may be made by laminating the acrylic composite sheet to a substrate material or by coextruding the acrylic composite onto a thermoplastic substrate or by extrusion coating onto e.g. a board substrate.
The production of filled acrylic articles by melt processing is more economical than casting and curing polymerisable compositions so the benefits of a filled acrylic material may be achieved at relatively low cost. The articles produced in this way have a solid look and feel and the natural look of stone may be achieved by selecting appropriate colours and particles.
The acrylic copolymer is a copolymer of methyl methacrylate. 50-99% by weight of the monomer units from which it is formed are derived from methyl methacrylate monomer. The remaining units comprise one or more copolymerisable alkyl acrylate or other methacrylate units, which may include functionalised alkyl acrylates. Preferred acrylates include C1-C8 alkyl acrylate, especially methyl, ethyl and butyl acrylate. Particularly preferred copolymers are derived from 70-95% methyl methacrylate and 5-30% of C1-C4 alkyl acrylate units. The resulting copolymer preferably has a Tg of at least 60xc2x0 C.
The acrylic copolymer may be made by any known polymerisation methods, especially solution, dispersion or bulk polymerisation. The polymer may contain residues of a polymerisation initiator, or other process additive. Preferred polymers also contain residues derived from polymerisation controllers such as chain transfer agents which are added to control molecular weight. Especially preferred additives include those chain transfer agents which also enhance the thermal stability of the polymer such as mercaptans, e.g. alkyl mercaptans. Suitable mercaptans and polymerisation methods using mercaptans as chain transfer agents in the production of acrylic polymers are already well known in the art.
The acrylic copolymer is mouldable in its molten form, i.e. it is of a grade of acrylic polymer which is generally classed as suitable for moulding. The melt flow index (MFI) of the polymer is preferably in the range 0.5-25 g/10 mins at 230xc2x0 C./3.8 kg (ASTM D-1238), more preferably 0.8-16 g/10 mins. Typically the molecular weight is between 60,000 and 180,000 preferably in the range 80,000 to 150,000 Mw, e.g. 90,000-120,000.
The mouldable composition comprises 20-89, preferably 40-80% wt of copolymer.
The mineral filler is a particulate mineral compound selected from alumina trihydrate (Al (OH)3) (ATH), magnesium hydroxide (Mg(OH)2), talc or barium sulphate. The particles preferably have an average particle size of 5-100 xcexcm, e.g. about 10-20 xcexcm. The mouldable composition and resulting article may contain 10-80 wt % of the filler, preferably 10-50 wt %. A mixture of more than one mineral compound may be used as the filler.
The melt-mouldable mixture and resulting moulded article also contain decorative particles, e.g. coloured particles which provide a decorative effect to the sheet. Suitable particles include polymeric particles such as coloured polyester or cross-linked acrylic particles of e.g. 5-5000 xcexcm average size. The decorative particles may contain colourants, opacifiers, fillers, plasticisers, stabilisers, lubricants etc. The mixture may contains 1-40% of such coloured particles, preferably 10-30% wt, more preferably 5-20% wt. A mixture of different coloured particles may be used to provide different appearances, e.g. to simulate the appearance of natural materials such as stone or granite.
The mixture may also contain other additives, such as stabilisers, toughening agents, lubricants, pigments and dyes. Suitable toughening agents include rubbers e.g. MBS rubber, core-shell particle toughening agents, e.g. graft copolymer core shell particles made from butyl acrylate and MMA, SEBS, Kraton, or SBR etc. The toughening agent may be present in the polymer at levels of 1-50%, more typically 3-25% A pigment or dye may be present in the polymer prior to mixing the polymer with the filler and other ingredients.
By melt-moulding we mean forming a shaped article by moulding under conditions, especially temperature, in which the polymer is molten and can be caused to flow and then cooling below the melt temperature such that the mixture retains the shape formed when the polymer was molten. Preferred such methods include extrusion and injection moulding. Preferably the moulding temperature is less than 270xc2x0 C. and is more preferably 150-220xc2x0 C. The moulding temperature must be maintained below the decomposition temperature of any of its constituents. For example, when ATH is used as the filler then the temperature should be maintained below the decomposition temperature of ATH.
In one preferred form we provide a process for forming an acrylic article comprising the steps of feeding to an extruder a mixture comprising:
(i) 20-89 wt % of an acrylic polymer which is a copolymer containing 50-99% wt of methyl methacrylate units and 1-50 wt % of a copolymerisable alkyl acrylate;
(ii) 10-80 wt % of a mineral filler composition selected from the group comprising alumina trihydrate, magnesium hydroxide, talc and barium sulphate;
(iii) 1-40 wt % of decorative particles;
and extruding the mixture at a temperature above 150xc2x0 C. through a die and subsequently cooling the extrudate to form a solid article.
In a second preferred form we provide a process for forming an acrylic article comprising the steps of forming a mixture comprising:
(i) 20-89 wt % of an acrylic polymer which is a copolymer containing 50-99% wt of methyl methacrylate units and 1-50 wt % of a copolymerisable alkyl acrylate;
(ii) 10-80 wt % of a mineral filler composition selected from the group comprising alumina trihydrate, magnesium hydroxide, talc and barium sulphate;
(iii) 1-40 wt % of decorative particles;
and injection-moulding the mixture at a temperature above 150 xc2x0 C into a mould to form a solid article.
A preferred form of article is a sheet, which may vary in thickness between  less than 1 mm and about 20 mm, e.g. 1-10 mm. Many other shapes of articles are possible, especially when using injection moulding techniques to melt mould the article.
We also provide a thermoformable article, preferably in the form of a sheet, comprising
(i) 20-89 wt % of an acrylic polymer which is a copolymer containing 50-99% wt of methyl methacrylate units and 1-50 wt % of a copolymerisable alkyl acrylate;
(ii) 10-80 wt % of a mineral filler composition selected from the group comprising alumina trihydrate, magnesium hydroxide, talc and barium sulphate;
(iii) 1-40 wt % of decorative particles.